Summary: It’s never too late to start applying five whys, even if you’re saddled with zillions of lines of legacy code. Just start asking why whenever you find a problem—you’ll automatically start fixing the 20% of underlying issues that cause 80% of your problems. Five whys was first discovered by Toyota—if it can work for cars, it can work for you.

In Part 1, I described how to use five whys to discover the root cause of problems, make corrections, and build an immune system for your startup. In Part 2, I explained how to get started with five whys and how IMVU built a startup immune system by applying five whys for months and years. In this final part, I’ll describe how to apply five whys to legacy startups.

It’s never too late to start asking why.

When I explain five whys to entrepreneurs and big-company types alike, I sometimes get this response: “Well, sure, if you start out with all those great tools, processes and TDD from the beginning, that’s easy! But my team is saddled with zillions of lines of legacy code and… and…”

So let me say for the record: we didn’t start with any of this at IMVU. We didn’t even practice TDD across our whole team. We’d never heard of five whys, and we had plenty of “agile skeptics” on the team. By the time we started doing continuous integration, we had tens of thousands of lines of code that wasn’t under test coverage.

But the great thing about five whys is that it has a Pareto principle built right in. Because the most common problems keep recurring, your prevention efforts are automatically focused on the 20% of your product that needs the most help. That’s also the same 20% that causes you to waste the most time. So five whys pays for itself awfully fast, and it makes life noticeably better almost right away. All you have to do is get started.

If it works for cars, it can work anywhere.

So thank you, Taiichi Ohno. I think you would have liked seeing all the waste we’ve been able to drive out of our systems and processes, all in an industry that didn’t exist when you started your journey at Toyota.

And I especially thank you for proving that this technique can work in one of the most difficult and slow-moving industries on earth: automobiles. You’ve made it hard for any of us to use the most pathetic excuse of all: “Surely, that can’t work in my business, right?” If it can work for cars, it can work for you.